This present invention is directed generally to an apparatus for conveying cut or fan-fold paper in a printer, and in particular, to a paper conveying apparatus which prevents slack from developing in the paper as it is conveyed in both the reverse and forward directions through the printer.
A conventional printer including an apparatus for conveying paper such as shown in Japanese Patent Application (OPI) No. 257871/1987 is illustrated in FIG. 1. The printer includes a platen 301, a sheet conveying roller 303, a print head 302, a first discharging roller 307 and a second discharging roller 308. Platen 301 and sheet conveying roller 303 form sheet conveying assembly 310. First discharging roller 307 and second discharging roller 308 form sheet discharging assembly 315. A printing sheet 100 is conveyed in the direction of the arrow G into the gap between platen 301 and print head 302 to a printing position (a). Print head 302 prints characters or data onto printing sheet 100 at printing position (a) as printing sheet 100 is conveyed through the printer. Printing sheet 100 is discharged by sheet discharging assembly 315. Sheet discharging assembly 315 is designed to discharge printing sheet 100 from the printer when the rear edge of printing sheet 100 has passed through the contact line of platen 301 and sheet conveying roller 303 (when the sheet conveying force applied to printing sheet 100 by sheet conveying assembly 310 has been suspended). The sheet conveying speed of sheet discharging assembly 315 is set to a value greater than that of sheet conveying assembly 310 while the sheet conveying force of the sheet discharging assembly 315 is set smaller than that of the sheet conveying assembly 310, so that printing sheet 100 is allowed to slip with respect to sheet discharging assembly 315. By this arrangement, printing sheet 100 is stretched tight when positioned between sheet conveying assembly 310 and sheet discharging assembly 315. This prevents slack from forming across paper sheet 100.
The conventional printer of FIG. 1 as described above was conceived based on the premise that printing sheet 100 is conveyed in the forward direction (sheet forward conveying direction) only. That is, no consideration is made for the case where a printing sheet is conveyed in the reverse direction (sheet reverse conveying direction), the direction opposite the forward direction.
When performing graphic printing operations using bit images and character printing operations using enlarged characters in combination, an ordinary printing method cannot be employed, because the capacity of the memory (buffer) in the printer or control software is limited. Therefore the following method should be employed. A plurality of lines are printed. Thereafter, the printing sheet is returned to the original position for another printing operation (i.e. graphics or enlarged characters). Thus, the printing of the sheet is accomplished as a whole. In this type of printing operation, it is essential to accurately convey the printing sheet (about 1/2 inch) in the reverse direction.
FIG. 2 illustrates the relationship between the amount of displacement of the printing sheet and the amount of rotation of the drive motor in the case where the sheet is conveyed in the forward direction and in the case where the sheet is conveyed in the reverse direction as carried out in a conventional printer of the type disclosed in Japanese Patent Application No. 257871/1987. In the case where printing sheet 100 is conveyed a distance D1, as shown in the first quadrant, the drive motor (not shown) should be rotated as much as d1 in the sheet forward conveying direction. Since the sheet conveying speed of sheet discharging assembly 315 is greater than the speed of sheet conveying assembly 310, sheet discharging assembly 315 tends to convey printing sheet 100 as much as the amount of displacement indicated by the broken line. Since the sheet conveying force of sheet discharging assembly 315 is smaller than that of sheet conveying assembly 310, printing sheet 100 is allowed to slip as much as S1 with respect to sheet discharging assembly 315. Hence, printing sheet 100 is conveyed to the point Pl from the origin while being stretched tight; that is, it is moved as much as D1.
In the case where the printing sheet 100 is conveyed only in the forward direction, even if there is an amount of play due to the backlash of the gear train between the drive motor, sheet conveying assembly 310 and sheet discharging assembly 315, the play is absorbed in one direction so the amount of rotation of the drive motor and the amount of displacement of printing sheet 100 are proportional to each other.
When the printer is to convey printing sheet 100 in the sheet reverse conveying direction after being conveyed in the forward direction, the drive motor is turned as much as dl in the reverse direction to convey printing sheet 100 in the reverse direction. However, printing sheet 100 does not move while the drive motor is turned in the reverse direction as much as a small amount of play (b), due to the backlash of the gear train between the drive motor, sheet conveying assembly 310 and sheet discharging assembly 315. Precisely stated, the amount of play between the drive motor and sheet conveying assembly 310 differs from the play between the drive motor and sheet discharging assembly 315. However, they are denoted by the same value (b) since they are generally small values (in FIG. 2, for convenience in description, (b) is about half d1, however, in practice, it is much smaller). After the play has been absorbed, sheet discharging assembly 315 displaces printing sheet 100 at a speed greater than the speed of sheet conveying assembly 310. As a result, sheet conveying assembly 310 displaces printing sheet 100 as much as D2 in the reverse direction, so that printing sheet 100 is moved through the origin and the point P2 to the point P3. On the other hand, while sheet discharging assembly 315 displaces printing sheet 100 in the reverse direction, printing sheet 100 is moved through the origin and the point P2 to the point P4. Thus, printing sheet 100 is slackened as much as the difference S2 between D2 and D3. In FIG. 2, D1&gt;D3&gt;D2, and printing sheet 100 differs in displacement in the forward and reverse directions, while the amount of rotation (d1) of the drive motor remains unchanged. Thereafter, when the drive motor is turned as much as d1, the amount of play (b) is absorbed in the opposite direction and the sheet is returned to the origin.
Printing sheet 100 is thus moved from the point P3 through the point P5 to the origin by sheet conveying assembly 310, whereas it is moved from the point P4 through the point P6 to the origin by sheet discharging assembly 315. As is apparent from the above description, when the printing sheet is conveyed in the reverse direction in a conventional printer, it is slackened, and as a result, the resultant print may be shifted in position, the printing sheet stained by ink or caught in the printer resulting in jamming.
In the case where print head 302 is of the wire impact dot type, the slack causes printing sheet 100 to be caught in the printer pushing the ink ribbon (not shown) located between print head 302 and platen 301 through the ribbon mask (not shown) positioned between print head 302 and platen 301 against print head 302, so that the wires of print head 302 are likely to be caught by the ink ribbon or broken. In addition, when the printing wires strike printing sheet 100 while it is raised away from platen 301, the printer emits a loud noise during printing.
Accordingly, it is desired to provide an apparatus for conveying paper in a printer which overcomes the disadvantages of the prior art device described above by providing an apparatus for conveying paper within a printer which eliminates slack during conveying in the reverse direction by applying tension to the recording medium when conveyed in either a forward or reverse direction thus preventing slack from occurring across the printing medium. Specifically, the problems which would be eliminated include the resultant print being shifted in position, the printing sheet being stained by ink or caught in the printer and the wires of the print head becoming damaged or broken.